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Full text of "The Cambridge Natural History"

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re persistently free for a very long period, and in some cases
ever become ankylosed with their vertebrae. But it should be
oted that in this group there is no approximation to the state
f affairs which exists in many lower Yertebrates, where there is
gradual transition between the ribs of the cervical and those of
le dorsal region of the vertebral column ; for that of the seventh
Ibs in Monotremes is smaller than those which precede it.

The Sternum. — All the Mammalia so far as is known possess
sternum. This is the bone,
p series of bones (sternebrae),
hich lies upon the ventral
urface of the chest, and to
hich the ribs are attached
slow. The development of
le sternum has been shown
> take place from the fusion of
le ribs below into two lateral
mds, one on each side; the
yproximation of these bands
•rms the single and unpaired
>ernum of most mammals.
ery considerable traces, how-
rer, of the paired state of the
lernal bones often exist ; thus
L the Sperm. Whale the first
Lece of the sternum is divided

ito     two      by     a      longitudinal

_      ,            °       .

L Vision,   and   the    Second    piece

longitudinally grooved. The
3velopment of the sternum
at of the fused ends of ribs is shown in a more com-
Lete condition, in some specis of Manis than in many other
lammals. Thus in M. tricuspis the last ribs of those which are
itached to the sternum are completely fused together into a
ngle piece on each side.1 As a general rule the last ribs
hich come into relation with the sternum do so only in an
uperfect way, being simply firmly attached at their sides to,
at not fused with, the last ribs which are definitely articulated
ith the sternum. Contrary to what is found in lower Verfce-
1 Ehlor's ZooL MisMvn,. \,

- 18.- Sternum and sternal ribs of the
Common Mole (Talpa, europaea), with.
the clavicles (cl) and humeri (i/) ; M,
N"fc size-   (Prom